<?php

/**
 * 翻转字符串
 *
 * 编写一个函数，其作用是将输入的字符串反转过来。输入字符串以字符数组 s 的形式给出。
 * 不要给另外的数组分配额外的空间，你必须原地修改输入数组、使用 O(1) 的额外空间解决这一问题。
 *
 * 示例一
 * 输入：s = ["h","e","l","l","o"]
 * 输出：["o","l","l","e","h"]
 *
 * 示例二
 * 输入：s = ["H","a","n","n","a","h"]
 * 输出：["h","a","n","n","a","H"]
 *
 * 1 <= s.length <= 105
 * s[i] 都是 ASCII 码表中的可打印字符
 *
 * 解题思路： 双指针
 * 重点是O(1)的空间复杂度
 *
 */

class Solution {

    /**
     * @param String[] $s
     * @return NULL
     */
    function reverseString(&$s) {
        $len = count($s);
        $left = 0;
        $right = $len - 1;
        while ($left < $right) {
            $tmp = $s[$left];
            $s[$left] = $s[$right];
            $s[$right] = $tmp;

            $left++;
            $right--;
        }
        return $s;
    }
}

$s = 'qazxswe';
$obj = new Solution();
$res = $obj->reverseString($s);
var_dump($res);